Poly(ethylene terephthalate) (PET) is a large volume resin finding application in the production of fibers, molded articles and packaging materials. Soft drink beverage bottles are made from resins comprised mainly of PET. Discarded beverage bottles do not degrade at acceptable rates by natural processes in the environment, and hence present both a litter and a disposal problem. One desirable way to lessen the amount of PET bottle refuse in the environment is to recycle the polymer into new PET bottles. Because of fortuitous contact of some post-consumer bottles with harmful substances, such as pesticides, a successful recycling technique will almost certainly mandate that the polymer be depolymerized to purified monomers.
A successful depolymerization process must be able to remove other substances that may be present in the resin such as isophthalic acid (IPA) and 1,4-cyclo-hexanedimethanol (used to control crystallinity), dyes and pigment (used to color certain bottles) and polymerization catalyst metals. Much of the PET produced is prepared from polymer grade terephthalic acid (PTA) rather than dimethyl terephthalate (DMT), and thus a suitable recycling process should produce PTA for processes that use it as a starting material.
U.S. Pat. No. 5,045,122 describes a saponification process for the degradation of PET into salts of terephthalic acid (TPA). A similar procedure that produces salts of TPA is described in U.S. Pat. No. 4,355,175, but the polymer is first dissolved in concentrated sulfuric acid--a step which generally has a detrimental effect on the ethylene glycol (EG) portion of the resin. Both of these references describe recovery of the terephthalic acid by neutralization of the salts with acid. Both of these processes are wasteful because acid and base are consumed and salts are created. Disposal of the salts thus has a negative effect on the environment. Accordingly, a need exists for an economical process for the recovery of both EG and TPA from PET in a manner that will not harm the environment.
U.S. Pat. No. 3,703,488 describes a method for degrading PET by mixing with TPA or ethylene glycol at elevated temperature. The resulting partially depolymerized material is then incorporated in the preparation of new resin. PTA is not produced by this method, and no means is provided to remove typical contaminants such as polymerization metal catalysts, dyes, pigments, other glycols and dicarboxylic acids, or fortuitous contaminants such as insecticides and gasoline.
U.S. Pat. No. 4,620,032 describes a related glycolysis process wherein the resin is partly depolymerized with EG and then hydrolyzed at elevated temperature and pressure, thus allowing for a shorter hydrolysis time compared to that required for resin not treated with EG. This method does not provide for the removal of contaminants such as polymerization metal catalysts, dyes, pigments, other glycols and dicarboxylic acids.
U.S. Pat. No. 5,101,064 describes a related process whereby the resin is treated with an excess of an alcohol having 6-20 carbon atoms, and ethylene glycol can be distilled from the mixture. Because transesterification is an equilibrium process, a large excess of the alcohol is generally required to obtain an acceptable yield of diesters of TPA (even though distillation of ethylene glycol favors the formation of the ester of the reactant alcohol because of the equilibrium), and, in order to obtain PTA, subsequent purification and hydrolysis steps would be required. A large alcohol recycle stream would be generated by such a process. Discharge of the alcohol or its by-products to the environment must be avoided, and this requirement makes this process relatively complicated and uneconomical.
DE 1,926,034 describes a process whereby PET is contacted with tall oil fatty acid at elevated temperature. Although this technique does not require a pressure vessel, it is also an equilibrium controlled process. Thus, a large excess of the fatty acid is required if an acceptable yield of TPA is to be obtained, and a large tall oil fatty acid recycle stream would be generated. Also, the process generates tall oil fatty acid esters of ethylene glycol. Thus, additional hydrolysis and purification steps would be required to recover the ethylene glycol. Discharge of the tall oil fatty acid or its by-products to the environment must be avoided, and this requirement makes this process relatively complicated and uneconomical. The process does not remove pigment colorants and other contaminants. Thus, a need exists for a process for the recovery of both PTA and EG from PET that will remove polymerization metal catalysts, dyes, pigments, fortuitous contaminants such as insecticides and gasoline, other glycols and dicarboxylic acids and that will not require the use of large volumes of environmentally undesirable reagents.
U.S. Pat. No. 4,578,502 describes an elevated temperature (about 177.degree.-288.degree. C.) and pressure (about 350-1500 psig) neutral hydrolysis process for the recovery of monomers from scrap PET. Related processes are described in U.S. Pat. No. 4,578,510 and U.S. Pat. No. 4,605,762. An attractive feature of these processes is that the ethylene glycol is easily recovered from relatively concentrated aqueous solutions by distillation.
U.S. Pat. No. 5,095,145 emphasizes that these type of neutral hydrolyses at elevated temperatures and pressures provide crude TPA that is unsuited for use as a monomer for PET. Further purification is accomplished by reslurrying the crude TPA in water and hydrogenating at elevated temperature and pressure in the presence of a noble metal catalyst. This treatment improves the color of the TPA, but the material still contains up to 100 ppm metals. None of these neutral hydrolysis techniques efficiently remove isophthalic acid, pigments, polymerization metal catalysts and fortuitous contaminants to the degree required for the production of PTA suitable for the use as a monomer for the production of PET. Complete hydrolysis is difficult to achieve.
Although some improvement in product quality might result from repeatedly redissolving the TPA at elevated temperature in water, filtering, cooling, recovering and washing the solid, many such steps would be required to achieve only a marginal improvement, and the process would thus be uneconomical. The difficulty arises from the similar solubility characteristics of many of the species present (such as IPA) to those of TPA and from the submicron size of many of the pigment particles which makes effective filtration difficult. The production of a uniform quality PTA from recycled polymer of varying composition would be difficult with processes based upon continuous neutral hydrolysis process. Thus, although neutral hydrolysis is an efficient way to produce crude TPA and readily recoverable ethylene glycol, a there is a need for the production of PTA of uniform high quality substantially free of IPA, pigments, fortuitous contaminants and polymerization metal catalysts.
Japanese Kokai Patent Application No. SHO 49[1974]-41329 describe the degradation of PET by superheated steam at 300.degree.-400.degree. C. at near ambient pressure. In this technique, TPA is vaporized as the polymer degrades and is collected by cooling the vapor stream. Since the TPA is produced in solution in the liquid melt before vaporization, the process is actually a distillation rather than a sublimation (sublimation is the passage from a solid state to a vapor state).
Other species vaporize along with the TPA, and these include 2-hydroxyethyl terephthalic acid (HETA), bis(2-hydroxyethyl) terephthalate (BHET), oligomers, ethylene glycol and ethylene glycol decomposition products. The formation of ethylene glycol decomposition products is expected when the polyester is pyrolyzed at temperatures in excess of 300.degree. C. as described in the literature (See, for example, March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure; McGraw-Hill, Inc.: New York, 1968; pp 756-757.).
A certain amount of decarboxylation occurs in the described process, although the extent is less when steam is present than when it is omitted. In addition to low observed conversions, the purity of the TPA produced by this method is typically only in the region of 70-80%. Acceptable TPA is obtained only after solvent washing and recrystalliztion using acetic acid. Thus this process is not only inefficient, but additional purification steps are required to provide pure TPA. Thus, a need still exists to provide for the efficient high yield production of polymer grade TPA and EG.
U.S. Pat. No. 3,330,863 describes a steam sublimation process for the purification of crude TPA produced from the oxidation of p-xylene. Related disclosures are British Nos. 1,163,665, 1,110,649 and 1,107,994, Belgian No. 719,187 and U.S. Pat. No. 3,362,989. A review article has been published (Bryant, H. S.; Duval, C. A.; McMakin, L. E.; Savoca, J. I. Chem. Eng. Progress 1971, 67, 69-75.). The primary impurity removed by this process is 4-carboxybenzaldehyde, and in some cases the process is performed in the presence of hydrogen gas and a noble metal catalyst to convert this impurity to 4-toluic acid. All of the processes involve heating the crude terephthalic acid in the presence of steam to a temperature sufficiently high to vaporize most of the TPA (about 315.degree. C.), filtration of the vapor phase to remove nonvolatile entrained solids and cooling to below the dew point of TPA. Steam is necessary to prevent anhydride formation.
The crude solid TPA is entrained in the vapor stream before it is vaporized. Crystallization is induced by contacting the hot vapor stream with cold TPA, liquid (generally water) or both, and generally the product is collected in cyclone separators or in filters. The steam sublimation process has never been demonstrated for the purification of crude TPA produced from post-consumer PET such as in the neutral hydrolysis with liquid water at elevated pressure.
Further, the teachings of Japanese Kokai Patent Application No. SHO 49[1974]-41329 suggest that undesirable degradation processes might occur because the crude product contains unhydrolyzed esters. Thus, a need exists to demonstrate that crude product derived from post-consumer or recycled PET such as the neutral hydrolysis of PET at elevated temperature and pressure can be converted in high yield to PTA, substantially free of HETA, BHET, oligomers, polymerization metal catalysts, IPA, dyes, pigments and fortuitous contaminants by a steam sublimation process.
The object of the present invention is to provide a process which converts poly(ethylene terephthalate) and its copolymers, including post-consumer products containing poly(ethylene terephthalate), into ethylene glycol and terephthalic acid. A further object of the invention is to provide ethylene glycol and terephthalic acid that is sufficiently pure to be suitable for the preparation of high quality high molecular weight poly(ethylene terephthalate) polymer suitable for use in clear bottles, fibers, etc., and that is substantially free of pesticide residues, gasoline residues, partially hydrolyzed esters, other glycols, other dicarboxylic acids, polymerization metal catalysts, dyes, pigments, and other fortuitous contaminants. A further object of the invention is to provide ethylene glycol and terephthalic acid in an economical manner without the use or production of organic reagents, products or inorganic salts and, therefore, in a manner that is safe and that has a minimal effect on the environment.